Gold & old alloys will soft solder to steel very easily.
Just as easy to solder it as to work with brass.
I use Lead/Tin soft solder for something like this as opposed to the newer Soft 'Silver Solder' (Tin/Silver alloy).
Either will work just as well.
The Tin/Silver soft solder stays forever bright white and I don't care for that bright white solder line betw the steel and the gold Facing, Bead or whatever attachment you are putting up.
Lead/Tin will oxidize in a short time especially if the steel part is Browned or rust blued. So no Silver line next to the gold.
A small thing but I guess it's one of those things called a detail.
If you do solder or inlay on a Gold (or Brass, Copper, etc) facing, inlay, bead or whatever to a sight or other part and then plan on Express Rust Browning or Rust Bluing it,,make sure you don't use one of the older formulas for the rusting process that contains any Mercury compounds (generally Mercury Bi-Chloride was the favorite).
Those rusting solutions work great, but the Mercury in them will Plate out onto any non-ferrous metal like your brass, gold, etc as you apply them. Especially if the parts are warm as in the Express Rust process.
The Mercury is plated very tenaciously to the brass or gold and does not just rub off. Carding will not usually remove it either.
Careful polishing can remove it but is a real pain and often damages surrounding new finish. If there is engraved detail in the inlay , that usually has to be re-cut.
Best to stick with a non-merc rusting soln in the first place.
Slow Rust soln's generally didn't have any merc compounds in them.
(It was used to touch up worn nickel plating in some instances on brass parts. Works pretty good for that.)